(1) Field of the Invention
The present invention relates to a hermetically sealed package useful for packaging foods or liquids and to method of producing the hermetically sealed package and more particularly, to hermetically sealed package having a top film which may be opened and closed repeatedly by pressure sensitive adhesive and to method of producing the hermetically sealed package.
(2) Description of the Related Art
In conventional method of closing a plastic container filled with an article such as jelly by means of a top film, a top film made of a plastic material of a type similar to that of container material has been used to be adhered by heat sealing to the flange portion surrounding the container. Though the coefficient of adhesion between the top film and the container varies depending on the temperature and pressure of heat sealing, it is generally so large (about 3 kg/25 mm width) that the top film cannot be torn off by means of fingers and needs to be cut with a cutting instrument to gain access to the content of the container. If the heat seal is weakened for the facility of opening, there will be caused the possibility of incomplete seal or accidental opening of the sealed package during storage or conveyance.
To obviate the defect of the conventional method, there has been employed a method of heat sealing a container at the flange portion thereof with a top film which has been provided with a layer coated with a wax (melt layer). (This method is called hot melt method.) This method utilizes cohesive failure of the melt layer caused by the difference between the adhesion force and cohesive force thereof and thereby makes it easy to separate top film away. However, this method has a defect of lacking in oil resistance.
In another method, a top film made of a material different from that of container and provided with an adhesive resin layer comprising vinyl acetate copolymer is heat sealed to the container. In this method, proper selection of pressure, time, and temperature of heat sealing permits to control the bonding strength (for example, 800 g/25 mm width to 1.2 kg/25 mm width) so that even infants or the aged can peel off the top film from the container with fingers. However, this method also has a defect that the heat sealed portion is so poor in heating tolerance that the package thus produced cannot be used for packaging articles which are to be subjected to heating treatment, such as retort treatment, since the heat sealed portion will be marred by pinholes generated during such treatment.
There is also known a package wherein the container portion thereof having a recessed portion and a flange portion surrounding the recessed portion is formed from a two-layered multilayer sheet, and a notch surrounding the boundary between the recessed portion and the flange portion is provided in the innermost layer of the multilayer sheet on the flange portion. The container is sealed with a top film sheet to form a hermetically sealed package. The top film comprises a lid portion and a tab portion extending from the rim of the lid portion, and the tab portion will not be adhered to the flange portion. When the hermetically sealed package is opened, the tab portion is pulled to be separated from the edge of the container, whereby the multilayer sheet is initially delaminated from the edge of the package and the delamination progresses to the notch of the container, with a part of the innermost layer outside the notch remaining adhered to the top film. By pulling progressively the tab portion, the above-mentioned part of the innermost layer is separated away with the top film from the container, and the hermetically sealed package is consequently opened. This method utilizes the difference between the adhesion strength between the layers of the multilayer sheet (for example, 300 to 1,200 g/25 mm width) and the adhesion strength between the container and the top film (for example, 3 kg/25 mm width).
The increase of small households, particularly one-person households, and the requirement for the rationalization of life presents an ever-increasing demand for such hermetically sealed packages as throwaway packages for various foods and liquids, sanitary articles, etc.
However, a consumer of hermetically sealed package does not always consume whole articles packaged at a time. In the conventional hermetically sealed packages, the top film once opened cannot be reused for preserving the remaining article.
Therefore, when the article to be packaged is a sanitary article, such as a pocket disinfecting cotton or disinfecting towel, or foods being apt to be easily damaged by dryness or dampness, the article should be packaged in an amount enough to a single usage. For example, some articles such as pickled ume are on the market in a form packaged individually. Such packaging however increases the cost of packaging and is also undesirable from the viewpoint of resource-saving. For all that, it is troublesome to transfer the remaining article into another container for preservation.
Therefore, there has been a demand for an inexpensive hermetically sealed package having a top film which can be reused for preserving the remaining article.
Nevertheless, there has been provided no throwaway, hermetically sealed package satisfying the demand as far as the present inventors know.